1. Field of the Invention
This invention pertains to structural and procedural aspects of offshore platforms useful in Arctic waters on a year-round basis. More particularly, it pertains to a gravity-type offshore platform structure having modular components readily assemblable in Arctic waters and preferably comprised principally of large floatable and ballastable reinforced concrete elements of honeycomb-type internal construction, which elements are assemblable by novel procedures to provide a range of particular platform structures each suited for particular sites of use within a wide range of water depths.
2. Review of the Prior Art and the Need Presented
The seas, bays and inlets on the margins of the Arctic Ocean, outside the realm of the permanent North Polar icepack, present especially difficult problems to those desiring to explore for and to develop the oil and gas reserves which are suspected and known to exist below these waters. These waters are often very shallow; in many areas 100 foot water depths are found 25 miles offshore. These waters are remote from major centers of industry and commerce. They are covered by sheet ice through November to May and by floe ice in June through August, in a typical year. Temperature variations are extreme.
Offshore drilling and production platforms useful in waters of these depths have been developed for use in less hostile environments. The factors noted above, in combination, mean that existing platform structures either cannot be used at all in the Arctic, or they can be used only for short periods annually when waters are free of ice. Existing platforms, if used, must either be moved into, used, and moved out of the area from remote locations between May and November, or they must be stored during ice periods in protected local harbors which, because natural harbors are virtually nonexistent, must be constructed at great cost. For these reasons, existing offshore platforms of conventional design have not been and are not likely to be used in the Arctic.
In recognition of the special problems posed by the Arctic environment, various innovative approaches to offshore operations have been proposed or implemented. Those approaches proposed include the use of a suitable platform and rig structure in a floating state during Arctic open water periods, use of the same structure on land during periods of ice formation and breakup, and use of the same rig on an ice sheet (without allowance for ice movement) during periods when the ice is of sufficient strength to support the structure; see U.S. Pat. No. 3,664,437. Other proposals seek to adapt platforms designed for warmer waters to Arctic conditions by the use of ice cutters and the like to the pylon of a monopod structure or the legs of a jack-up structure; see, for example, U.S. Pat. Nos. 3,669,052, 3,693,360 and 3,696,624. Still other proposals involve the use of massive moored floating platforms of conical or bell-like shape capable of being heaved buoyantly to break and stand against encroaching ice. Yet another proposal involves a massive unitary fixed platform having a conical or hourglass configuration at and adjacent its waterline for causing encroaching ice to ride up on the structure and so break; see U.S. Pat. No. 3,972,199. Other proposals involve combinations and variations of the described proposals.
To date, none of the proposals reviewed above has been adopted in support of offshore operations in the Arctic. The reasons are varied. In some cases, the proposals are not suited to the shallow waters of interest. In other cases, the costs of construction, placement and operation of the proposed structures are unattractive. In some cases, the proposed structures are not sufficiently adaptable to varying sites of use to warrant the requisite investment.
The innovation which has been adopted to date in support of offshore Arctic operations is the artificial island. Artificial islands are constructed in shallow water from rock, gravel and sand to provide an operations site capable of standing against extreme local environmental forces, notably those due to moving sheets or floe ice. While satisfactory and economically feasible in some circumstances, artificial islands have practical limitations on their utility. They are not movable. They are costly to construct; construction costs rise sharply with increasing water depth. Gravel and rock are not naturally readily available in many areas of interest; ready availability of adequate supplies of these materials directly affects the cost of constructing an artificial island. Proposals to overcome these limitations of artificial islands by the use of man-made year-round ice islands have their own limitations and have not been adopted.
It is thus seen that a need exists for a structural and procedural system which provides an Arctic offshore operations platform, such as an oil or gas drilling or production platform. Such a platform should be versatile, i.e., capable of use directly, or without substantial or costly modification, in waters of various depths. The platform should be readily movable to enable it to be used in different places over its useful life which should be long. The system should be adaptable to varying sea floor soils and soil conditions with minimal dredging or other preparation of the sea floor site. The platform must be capable of use year-round in the face of forces, notably ice-generated forces, tending to move the platform from its site of use. The platform should be capable of being readily and economically fabricated in existing construction facilities remote from the Arctic, and moved effectively and efficiently, without undue hazard, to Arctic waters where it can be readily installed without reliance on costly special equipment or procedures. The materials used in constructing the platform should be readily available, of reasonable cost, and compatible with the hostile Arctic environment. The basic platform structure also should be compatible with a wide range of superstructure arrangements, thus enabling the platform to be used by different owners and operators who have their own preferences for functional equipment sets and layouts, and to be used for differing purposes such as exploration drilling, production drilling, and production from completed production wells, among other purposes. Further, the platform structure and its method of installation must be compatible with and protective of indigenous marine life and related environmental standards which are stringent in the Arctic.